by Modern life depends on electricity and electrical devices, from cars and buses to phones and laptops, to the electrical systems in homes. Behind many of these devices is a type of energy storage device, the supercapacitor. My team of engineers is working on making these supercapacitors even better at storing energy by studying how they store energy at the nanoscale.
Supercapacitors are energy storage devices, like batteries. They charge faster than batteries, often in a few seconds to minutes, but generally store less energy. They are used in devices that require bursts of energy to be stored or supplied over a short period of time. In your car and in lifts, they can help recover energy during slow down braking. They help meet the fluctuating demand for energy in laptops and cameras, and stabilize the energy loads in electrical grids.
Batteries function through reactions in which chemical species give or take electrons. Supercapacitors, in contrast, do not rely on reactants and are like a charged sponge. When you dip a sponge in water, it soaks up the water because the sponge is porous – it has empty pores where water can be absorbed. The best supercapacitors set the largest charge per unit volume, meaning they have a high energy storage capacity without taking up too much space.
In research published in the journal Proceedings of the National Academy of Sciences in May 2024, my student Filipe Henrique, collaborator Pawel Zuk and I describe how ions move in a network of nanopores, or tiny pores that are not just a nanometer wide. This research could one day improve the energy storage capabilities of supercapacitors.
All about the pores
Scientists can increase a material’s capacitance, or ability to store charge, by making its surface porous at the nanoscale. A nano-jar material can have a surface area of as much as 20,000 square meters (215,278 square feet) – the equivalent of about four football fields – and weigh just 10 grams (one-third of an ounce).
For the past 20 years, researchers have studied how to control this porous structure and the flow of ions, which are tiny charged particles, through the material. Understanding the flow of ions can help researchers control the rate at which a supercapacitor charges and discharges energy.
But researchers don’t know exactly how ions flow in and out of porous materials.
Each pore is a small hole in a sheet of porous material filled with positive and negative ions. The opening of the pore connects to a reservoir of positive and negative ions. These ions come from an electrolyte, a conducting fluid.
For example, if you add salt to water, each salt molecule separates into a positively charged sodium ion and a negatively charged chloride ion.
When the pore surface is cut, seepage flows from the reservoir into the pore or vice versa. If the surface is positively charged, negative ions flow into the pore from the reservoir, while positively charged ions leave the pore and are repelled. This flow creates capacitors, which hold the charge in place and store energy. When the surface charge is released, the ions flow in the reverse direction and the energy is released.
Now, imagine that a pore divides into two different branched pores. How do the ions flow from the main pore to these branches?
Think of ions as cars and pores as roads. Traffic flow on one road is simple. But at an intersection, you need rules to prevent an accident or a traffic jam, so we have traffic lights and roundabouts. However, scientists do not fully understand the rules that ions follow when they flow through a junction. It could help researchers understand how a supercapacitor will break these rules.
Modifying the law of physics
Engineers generally use a set of physics laws known as “Kirchoff’s laws” to determine the distribution of electrical current across a junction. However, Kirchhoff’s circuit laws were derived for electron transport, not ion transport.
Electrons only move when there is an electric field, but ions can move without an electric field, by diffusion. In the same way that a pinch of salt slowly dissolves throughout a glass of water, ions move from more concentrated areas to less concentrated areas.
Kirchhoff’s laws are similar to accounting principles for circuit junctions. The first law states that the current entering a junction must equal the current leaving it. The second law states that voltage, the pressure that pushes electrons through the current, cannot change suddenly across a junction. Otherwise, it would create an additional current and disturb the balance.
Since ions also move by diffusion and not just by using an electric field, my team modified Kirchhoff’s laws to fit ionic currents. We replaced the voltage, V, with an electrochemical voltage, φ, which combines voltage and diffusion. This modification allowed us to analyze pore networks, which was previously impossible.
We used a modified Kirchoff’s law to simulate and predict how ions flow through a large network of nanopores.
The road ahead
Our study found that splitting current from pore to junction can slow down how fast charged ions flow into the material. But that depends on where the split is. And how these pores are arranged throughout the materials also affects the charging speed.
This research opens new doors to understanding the materials in supercapacitors and developing better ones.
For example, our model can help scientists simulate different pore networks to see which best matches their experimental data and optimize the materials they use in supercapacitors.
Although our work focused on simple networks, researchers could apply this approach to much larger and more complex networks to better understand how the porous structure of a material affects its performance .
In the future, supercapacitors may be made from biodegradable materials, flexible power wearables, and may be customized through 3D printing. Understanding ion flow is an important step in improving supercapacitors for faster electronics.
This article is republished from The Conversation, a non-profit, independent news organization that brings you reliable facts and analysis to help you make sense of our complex world. It was written by Ankur Gupta, University of Colorado Boulder
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Ankur Gupta receives funding from the NSF.
